Isothermal electromagnetic apparatus



W. FONDILLER ISOTHERMAL ELECTROMAGNETIC APPARATUS Oct. 26, 1954 2Sheets-Sheet 1 Filed Dec. 12, 1950 INi/ENTOR W/LL/AM POND/LL ER A7'7'ORNEY 1954 w. FONDILLER ISOTHERMAL ELECTROMAGNETIC APPARATUS 2Sheets-Sheet 2 Filed Dec. 12, 1950 FIG. .3

' lNVE/VTOR WILL/AM FUND/LL67? sup/1Y1 3 ATTORNEY Patented Oct. 26, 1954ISOTHERMAL ELECTROMAGNETIC APPARATUS William Fondiller, New York, N. Y.,assignor to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application December 12, 1950,Serial No. 200,475

12 Claims. 1

This invention relates generally to electromagnetic apparatus and moreparticularly to mounting structure therefor.

One object of the invention is to provide means for designing relays andother electromagnetically operated apparatus for improved performance.

Another object is to enable such apparatus to be designed for greatereconomy.

In the interest of both performance and economy, electromagnets for usein telephone central office equipment are usually made as small aspracticable. There are, however, several considerations which usuallyrequire electromagnets to be made larger than they would be ifperformance and economy were the only considerations. In the firstplace, the temperature of the electromagnet due to heat generated in itsoperation should be limited so as not to make it difficult for amaintenance man to Work on other equipment mounted nearby. In the secondplace, the temperature of the electromagnet under trouble conditionsshould be limited so that the electromagnet does not constitute a firehazard. Finally, electromagnetically operated equipment in telephonecentral offices usually undergoes wide temperature variations whichaffect its performance adversely. The first two considerations have theeffect of limiting the current which may be applied to the electromagnetwindings to achieve the required magnetomotive force, while the thirdhas the effect of requiring the electromagnets to be designed to producesufficient magnetomotive force to assure successful operation underadverse conditions such as, for example, at elevated temperatures. Allthree have the effect of requiring more than the optimum number of turnsin the electromagnet windings and, in order to provide an adequate heatradiating surface, relatively large electromagnets result.

In accordance with a principal feature of the present invention, thecores of the electromagnets of electromagnetically operated apparatusare maintained at a substantially constant low temperature. Heat isthereby conducted readily from the electromagnets themselves and theyare thereby also maintained at a substantially constant low temperature.Thus, not only are winding current limitations relaxed but also thetotal number of ampere-turns necessary to assure successful operationunder all circumstances is reduced. Smaller coils can be used andelectromagnetically operated apparatus can be designed for optimumperformance and economy.

In accordance with another feature of the invention, electromagnets arecooled by means of their mounting structure, which is composedprincipally of heat-conducting material and which includes conduits fora suitable cooling fluid located adjacent to the portions of thestructure upon which the apparatus is mounted. The conduits are integralwith the mounting structure to promote efficient cooling. Theelectromagnets are thereby refrigerated efiectively withoutnecessitating either a radical change in their design or therefrigeration of the entire room in which they are located, thuspermitting optimum electromagnet design at a minimum of expense.Flexibility is also achieved, as different bays of equipment can readilybe maintained at different fixed temperature levels.

In accordance with still another feature of the invention, a directmetallic heat-conducting path is provided between the core of eachmounted electromagnet and a cooling fluid channel. Highly efiicient heatconduction is thereby provided and the heat-generating electromagnetsare cooled with a minimum effect on the surrounding air and apparatus.

Other objects and features of the invention will appear from a study ofthe following detailed description of several specific embodiments. Inthe drawings:

Fig. 1 shows a refrigerated relay rack the temperature of which iscontrolled by a diaphragm valve;

Fig. 2 is a cross-sectional detail of the rack shown in Fig. 1, takenalong the line 2-2; and

Fig. 3 shows a refrigerated relay rack the temperature of which iscontrolled by a motor driven valve.

As has been noted, for purposes of both performance and economy it isdesirable to make electromagnets for use in telephone central oifices aswell as those for power applications as small as practicable. Smallrelays, for example, are generally both faster and cheaper than largerrelays and also take less mounting space. In order to design a relay ofminimum size and cost and yet maintain the magnetomotive force requiredto operate the relay under the most adverse conditions, it is generallydesirable to minimize the number of turns and the diameter of the wirewith which the electromagnet coil is wound.

A number of factors enter into the design of such electromagneticapparatus as relays which tend to increase the number of turns requiredin the electromagnet windings. As has been pointed out, two of these aretemperature limits, above which the electromagnet and its associatedapparatus should not be permitted to rise. In continuous operation, itis desirable that the maximum temperature of relays be such that aworkman working on the mounting structure or framework will not besubjected to a temperature of more than about 150 F. It is thereby madecertain that the framework can be worked on at all times without dangeror serious discomfort to persons so employed. Translating frametemperature to coil temperature, it is generally found under existingmounting practices that this condition will obtain if the temperature ofthe winding does not go above 225 F.

The other temperature limitation in relay desi n for trouble conditions,at which time it is essential that the electromagnet shall notconstitute a fire hazard. In that connection, it is generally found thata relay will, be safe if the temperature of the winding never exceeds360 F. Above that temperature, insulation burns and more and more turnsare short-circuited, causing both an operating failure and a firehazard.

As has been noted briefly, top temperature limitations make it necessaryto design electroma nets so that their operating currents never exceedthe corresponding limits. In order to achieve the required magnetomotiveforce, the turns of the windings must be increased. The resulting relayis slower, more expensive, and more bulky than would otherwise benecessary.

The other factor in relay design which is of interest in connection withthe present invention is the wide variation in temperatures to which therelays are likely to be subjected. In unattended telephone centraloiiices, for example, the ambient temperature may reach as high as 120F. and may go as low as 40 F. In continuously operated relays, thewinding temperature may exceed the ambient temperature by as much as 125F., with the mounting structure temperature at an intermediate value.Wide temperature variations are generally undesirable because of theiradverse effect upon relay performance. As the winding temperatureincreases, the time required for the relay to operate also increases,until finally a point is reached where the relay will fail to operate. Arelay must, therefore, gen rally be designed with a sufficient margin ofmagnetomotive force so that it will operate effectively not only ataverage operating temperatures but also at the highest temperatures towhich it is likely to be subjected. Thus, more than the optimum numberof ampereturns must generally be provided, resulting both in a largerrelay winding and in a larger operating current than would otherwise benecessary.

Tests have indicated that a large part of the heat generated in relaywindings is conducted away through the magnetic cores to the supportingstructure. In accordance with principal features of the presentinvention, as heretofore discussed, the relay and its mounting structureare both maintained at a substantially constant low temperature. Adirect metallic heat-conducting path is provided between each windingcore and a coolant channel. Heat is thereby readily conducted away fromthe relay windings, and the windings are maintained at a substantiallyconstant temperature. The constant low tempera ture permits relays to bedesigned for optimum performance and economy and allowance for widetemperature variations is no longer required.

If, for example, the mounting plate temperature is reduced to about 40F., and maintained within a few degrees of that value, substantialeconomies can be realized and more satisfactory operating conditionsmaintained. The upper current limitations for relay windings arerelaxed, since more heat can be dissipated in the windings withoutcausing either the winding temperature or the frame temperature toexceed the predetermined limits. Smaller electromagnets can be employed,and relays will not only be cheaper and occupy less mounting space butalso will require less magnetomotive force, for operation, since themost adverse operating conditions have been ameliorated. Among thevariables normally affecting electromagnet performance, there should beconsidered number of turns, winding resistance, battery voltage, lengthof circuit loop, and ambient temperature. All of these are allowed tovary within limits because of the cost of controlling all of them toexact values. Of these, the winding resistance experiences the mostimportant change under extremes of operating conditions. If, forinstance, the relay is designed to operate on the longest circuit loop,it is likely on short loops to receive an operating current which Willheat it beyond the tolerable limit. The principles of the presentinvention permit the operation of the relay under the most adversecircuit and ambient temperature conditions to be as favorable as underthe best conditions. In other words, the present temperature controlmethod stabilizes relay performance and makes it independent of bothoperating and environmental conditions.

Magnetomotive force requirements for relays mounted in accordance withthe present invention are also reduced by reason of the elimination ofwide temperature variations. Since the relay temperature is heldpractically constant along with that of the mounting structure, there isno need for providing more ampere-turns than are required to operate therelay in the desired time interval. The reduction in magnetomotive forcerequirements made possible by the reduction and stabilization of thetemperature of relay windings is advantageous in a number of respects.Not only are fewer turns required to achieve the required magnetomotiveforce, but also the reduction may, if desired, be turned into a savingin direct-current power through reduced operating currents. Since thealternating-current power required for refrigeration is much cheaperthan direct-current power derived from storage batteries, a substantialreduction in cost can be realized in this manner. Other advantages madepossible by the reduced magnetomotive force requirements can be realizedto the greatest degree if a fairly large ampere-turn margin remains inthe redesigned relay. The presence of a large margin of ampere-turns in.excess of actual operating requirements makes operation faster and morereliable. Maintenance costs are reduced because very little readjustmentof relays will be required.

In accordance with the features of the present invention, theabove-noted improvements in electromagnet and relay design are madepossible without introducing radical design requirements and withoutreducing the room temperature appreciably. As previously noted, conduitsintegral with the supporting structure are provided adjacent to theportions of the structure upon which the relays or otherelectromagnetically operated equipment are mounted. The conduits carry asuitable refrigerant and, in the coolin fluid circuit,temperature-sensitive means is provided to maintain both the mountingand the apparatus temperatures substantially constant by controlling theflow of th refrigerant. Pipes carrying the refrigerant areheat-insulated at all points except where they come into contact withthe mounting elements. The advantage of refrigerating only the mountingunits instead of the entire equipment room is that a minimum amount ofpower is required, since the heat transfer from the relay core andmounting will take place at the point of maximum temperature differenceand by the most effective heat transfer method, metallic conduction.Individual refrigerating units could be designed for each bay ofequipment, making it possible to maintain different fixed temperaturesfor different units, depending on the results desired.

When a bay of relay or other electromagnetically operated equipment iscooled in accordance with the principles of the present invention, careshould be taken to maintain the temperature of the refrigerated partshigher than a certain critical temperature, i. e., the dew point. Thedew point is the temperature at which moisture will begin to condense ata given air temperature, barometric pressure, and relative humidity.From a study of psychrometric tables, it is apparent that lowering thetemperature of an object appreciably below the ambient temperature willcause condensation unless the relative humidity is kept at a low figure.For example, at a room temperature of 80 F., a barometric pressure 30inches of mercury, and a relative humidity of 80 per cent, the dew pointis 73 F. This will not allow a material margin for the cooling ofoperating parts by refrigeration. However, if the relative humidity isreduced to 15 per cent, other conditions remaining the same, the dewpoint drops to 28 F. This will allow cooling approximately 50 degreesbelow room temperature without causing condensation.

The winding temperature will, in general, greatly exceed the ambienttemperature and can be disregarded from the standpoint of condensation.The temperature of the equipment, which, in accordance with the presentinvention, is maintained substantially constant by thermostatic controlof the refrigeration unit, should always be greater than the dew point.In practice, it need exceed the dew point by only 2 or 3 F. This canreadily be accomplished with widely varying ambient temperatures bycontrolling the humidity. Since, in modern installations, electromagnetsare enclosed for dust protection in a way which easily lends itself tochemical dessication of the air, the relative humidity can be kept at 15per cent or less with little diiiiculty. Under enclosed conditions, theheat radiation from relays is impaired, tending to raise their operatingtemperatures and thus further enhancing the merit of cooling them inaccordance with the teachings of the present invention.

Still another advantage which may be derived from the employment offeatures of the present invention is that a solder-through insulatingenamel may be used on the relay windings. When such an enamel is used,it need not be scraped away for connections to be made to the windings.It has been found, however, that such enamel deteriorates at hightemperatures, and the heat generated in relay windings usually prohibitsits use thereon. If the winding temperatures are, in accordance with thepresent invention, maintained at a suficiently low level, a suitablesolderthrough enamel may be used, with all the accompanying benefits ofeconomy and simplicity.

A specific embodiment of the invention is illustrated in Fig. 1.Referring to Fig. 1, a refrigerated relay rack is shown which comprisesa large number of mounting plates ll supported by a pair of verticalchannels 12. Mounting plates II are mounted parallel to and in contactwith each other and extend substantially horizontally. While only a fewsuch plates I l are shown for reasons of clarity, in practicalinstallations they will normally extend from the bottom to the top ofthe bay of which they form a part. A large number of relays 13 aremounted in rows on mounting plates I l and extend substantiallyperpendicularly thereto.

A refrigerant-carrying conduit I4 is provided in each mounting plate I lparallel and closely adjacent to the row of relays mounted thereon. Eachconduit [4 is shown, by way of example, as being rectangular in crosssection and is preferably fabricated as part of the mounting plate Hitself. At both ends of each conduit [4, openings are provided in thebase of mounting plate i l for the entry and exit of the coolant.

The conduits 14 are connected substantially in parallel by a pair ofvertical pipes [5, which are located to the rear of mounting plates Hwithin the respective channels l2. At regularly spaced intervals, thevertical pipes [5 are connected to conduits M by short connecting pipes16. Connecting pipes l6 enter conduits 14 through the openings providedat each end in mounting plates H. Vertical pipes 15 and connecting pipesI 6 are provided with a suitable heat-insulating covering to avoidunnecessary cooling of the surrounding air, and each vertical pipe [5 iscapped at both top and bottom.

A suitable refrigerant is applied to the pipingconduit system at thelower right-hand corner of the structure. An input pipe IT is tappedinto the right-hand vertical pipe [5 below the lowermost mounting plateI l. Included along input pipe I! are a cut-off valve l8 and a drainvalve i9, drain valve 89 being situated between cut-off valve !8 andvertical pipe l5. Egress for the refrigerant is provided by an outputpipe 20 at the upper lefthand corner of the structure. Output pipe at istapped. into the left-hand vertical pipe i5 above the uppermost mountingplate 1 l. A cut-off valve 2! is provided in output pipe 20, which,along with cut-off valve 58, may be used to isolate the refrigeratingsystem for the particular bay of equipment from those of the adjoiningbays.

The refrigerant, which may, by way of example, be brine, is suppliedunder pressure and is thereby circulated through the conduits M. A crosssection of a portion of the Fig. 1 structure appears in Fig. 2, showingdetails of the structure of mounting plates II and conduits l4 and themode of connection between vertical pipes I5 and conduits M.

The entire relay mounting structure is maintained at a substantiallyconstant temperature by controlling the flow of the cooling fluid. Athermostatically controlled diaphragm valve 22 is provided in outputpipe 20 between cut-off valve 2| and vertical pipe 15. Atemperature-sensitive element 23 is connected to diaphragm valve 22 bysuitable tubing 2a and is situated within the top portion of theleft-hand vertical pipe 15. Sensitive element 23 is filled with avolatile liquid, and as the temperature of the refrigerant surroundingelement 23 rises, the liquid volatizes, thus increasing the pressurethrough tube 24 and opening diaphragm valve 22. As the temperatur of therefrigerant decreases, pressure in tube 24 is reduced and valve 22 isclosed.

The positions of sensitive element 23 within the uppermost portion oftheleft-hand vertical pipe l and of diaphragm valveat the outputterminal of the system insure the most efiective operation of therefrigerating system. The temperature of the refrigerant will beincreased to the greatest extent at the output end of the system, andchanges in the flow of the refrigerant will, therefore, accuratelyreflect changes in the refrigeration requirements imposed by thecondition of the relays l3.

Another embodiment of the invention is illustrated in Fig. 3. The relayrack and the incorporated refrlgeration equipment are substantially thesame as in the Fig. 1 system and will not be redescribed. The principaldifference resides in the thermostatic control mechanism, which may beused as one alternative to that shown in Fig. '1.

In Fig. 3, the flow of the refrigerant is controlled by a motor-drivenvalve 28, which is located in series with output pipe between cut-offvalve 2i and the left-hand vertical pipe I 5. A temperature-sensitiveelement 29, corresponding to temperature-sensitive element 23 in Fig. 1,is located within the upper portion of the left-hand vertical pipe l5and surrounded by the refrigerant. Sensitive element 25) difiers fromsensitive element 23 in Fig. 1, however, in that it contains atemperature-sensitive electrical resistance, 1. e., a resistor theresistance of which either increases or decreases with temperature.Leads connected to the ends of the temperature-sensitive resistance areinsulated and are brought out through the cap at the top of theleft-hand pipe l5. From there, the leads are used to connect thetemperature-sensitive resistance into a bridge circuit, the other threearms of which comprise two resistors and 3! and the resistance arm of aslide-wire rheostat 32. A battery or other suitable directcurrent.source 33 is connected between two 0pposite corners or" the bridgecircuit, and a polarized relay 34 is connected between the other two.

A small series-type valve-operating motor 35 is provided with a shaft36, which is, in turn, coupled both to valve 23 and the contact arm ofrheostat 32. The resistance arm of rheostat is in the form of an arc,and the eiiective resistance presented by it to the bridge circuit iscontrolled by the rotation of shaft 36. Motor 35 is provided with a pairof field coils which are connected so that, when energized, each turnsthe armature in the opposite direction from the other. Current issupplied to motor 35 from an alternating-current source 3?, one side orwhich is connected to one side of both field coils and the other side ofwhich v is connected to the other side of one coil or the other,depending on the direction of the unbalance of the bridge circuit.

Polarized relay 35 includes, as its principal operating part, athree-terminal switch. The center terminal is connected to a contactspring and to the previously-mentioned other side of alternating-currentsource 31. The other two terminals are connected to contacts and torespective field coils of motor 35.

When the temperature of the refrigerant surroundingv sensitive element29 departs from apredetermined value, the bridge circuit becomesunbalanced, causing relay 34 to operate. When the refrigeranttemperature is too high, the direction of bridge unbalance causes relay3% to complete the circuit of motor 35 which turns the armature andshaft 35 in the direction to open valve 38. The flow-of cooling fluid isthus increased, and the bridge is balanced once more by the action ofshaft 36 in adjusting the resistance presented by 8. rheostat 32'. Whenthe temperature of the refrigerant is. too low, the operation isreversed. The outer contact of relay 34 is closed, and the armature ofmotor 35 is caused to turn in the opposite direction. Valve 2% isclosed, and the action of rheostat. 32 tends to rebalance the bridgewhen the correct valve opening is reached.

It will be observed that the advantages of the present invention insofaras improved relay periormance is concerned may be secured by redesigningthe relays themselves so that the cooling fluid passes through conduitsin the core of each electromagnet. The mounting structures which havebeen disclosed are, however, preferable since they are relatively simpleand inexpensive, and permit conventional relay design principles to beemployed.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel hollowheatconducting cross-members extending between saidsupporting members, aplurality of substantially parallel heat-conducting mounting plates eachsecured between and in contact with a pair of successive hollowcross-members, means to secure a plurality of electrical relays to eachor said mounting plates with a direct heat-conducting path between thecore of each relay and the nearest one of said hollow cross-members,means to circulate cooling fluid through said hollow crossmembers toreduce the temperature or" said mounting plates to a predeterminedlevel, and means to maintain the. temperature of said mounting platesaccurately at said predetermined level by regulating the flow of coolingfluid.

2. An isothermal relay rack which comprises, in combination, a pair ofvertical supporting members, a plurality of horizontal hollowheatconducting cross-members extending between said supporting members,a heat-conducting mounting plate facing at right angles to both saidsupporting members and said cross-members secured between and in contactwith each successive pair of cross-members, means to secure a horizontallayer of electrical relays to the same side of each said mounting platewith a direct heat-conducting path between the core of each relay and atleast one of said hollow cross-members, means to circulate cooling fluidthrough said hollow cross-members to reduce the temperature or saidmounting plates to a predetermined level, and means to maintain thetemperature of said mounting plates accurately at said predeterminedlevel by regulating the flow of cooling fluid.

3. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of heat-conducting mounting platesmounted edge to edge, each of said mounting plates extending betweensaid supporting members, a plurality of substantially parallelheatconducting conduits extending between said supporting members, eachof said conduits being secured to one side of a respective one of saidmounting plates, means to secure a plurality of electrical relays tosaid mounting plates in substantially parallel. rows with a directheat-conducing path between the core of each relay and at least one ofsaid conduits, one row to each of said mounting plates, means tocirculate cooling fluid through said conduits to reduce the temperatureof said mounting plates to a predetermined level, and means to maintainthe temperature of said mounting plates accurately at saidpredelizierrgined level by regulating the flow of cooling 4. Anisothermal relay rack in accordance with claim 3 in which all of saidconduits are secured to the same side of said mounting plates and saidrelay securing means is adapted to secure the relays to the same side ofsaid mounting plates as said conduits, with the terminals of the relaysprojecting through to the other side of said mounting plates.

5. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel hollowheatconducting cross-members extending between said supporting members,a plurality of substantially parallel heat-conducting mounting plateseach secured between and in contact with a pair of successive hollowcross-members, means to secure a plurality of electrical relays to eachof said mounting plates with a direct heat-conducting path between thecore of each relay and at least one of said hollow cross-members, meansto circulate cooling fluid through said hollow crossmembers to reducethe temperature of the rack to a predetermined level, and meansincluding a temperature-sensitive element thermally coupled to saidhollow cross-members to maintain the temperature of the rack accuratelyat said predetermined level by regulating the flow of cooling fluid.

6. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel hollowheatconducting cross-members extending between said supporting members,a plurality of substantially parallel heat-conducting mounting plateseach secured between and in contact with a pair of successive hollowcross-members, means to secure a plurality of electrical relays to eachof said mounting plates with a direct heat-conducting path between thecore of each relay and at least one of said hollow cross-members, a pairof conduits connecting said hollow cross-members substantially inparallel, means to circulate cooling fluid through said hollowcross-members and said conduits to reduce the temperature of the rack toa predetermined level, a valve at one side of said hollow cross-membersto regulate the flow of cooling fluid, and means including atemperature-sensitive element thermally coupled to said hollowcross-members to maintain the temperature of the rack accurately at saidpredetermined level by controlling the opening in said valve.

7. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel hollowheatconducting cross-members extending between said supporting members,a plurality of substantially parallel heat-conducting mounting plateseach secured between and in contact with a pair of successive hollowcross-members, means to secure a plurality of electrical relays to eachof said mounting plates with a direct heat-conducting path between thecore of each relay and at least one of said hollow cross-members, a pairof conduits connecting said hollow cross-members substantially inparallel, means to circulate cooling fluid through said hollowcross-members and said conduits to reduce the temperature of the racktoa predetermined level, and means including a temperature-sensitiveelement thermally coupled to said hollow cross-members to increase theflow of cooling fluid whenever the temperature of the rack rises abovesaid predetermined level and to decrease the flow of cooling fluidwhenever the temperature of the rack falls below said predeterminedlevel.

8. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel hollowheatconducting cross-members extending between said supporting members,a plurality of substantially parallel heat-conducting mounting plateseach secured between and in contact with a pair of successive hollowcross-members, means to secure a plurality of electrical relays to eachof said mounting plates with a direct heat-conducting path between thecore of each relay and at least one of said hollow cross-members, a pairof conduits connecting said hollow cross-members substantially inparallel, means to circulate cooling fluid through said hollowcross-members and said conduits to reduce the temperature of the rack toa predetermined level, a diaphragm valve at one side of said hollowcross-members to regulate the flow of cooling fluid, and means tomaintain the temperature of the rack accurately at said predeterminedlevel by controlling the opening in said valve comprising atemperature-sensitive element in the form of a container of volatileliquid thermally coupled to said hollow crossmembers and coupling meansbetween said temperature-sensitive element and said valve, whereby theopening in said valve varies with the gas pressure produced by saidtemperature-sensitive element.

9. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel hollowheatconduoting cross-members extending between said supporting members,a plurality of substantially parallel heat-conducting mounting plateseach secured between and in contact with a pair of successive hollowcross-members, means to secure a plurality of electrical relays to eachof said mounting plates with a direct heat-conducting path between thecore of each relay and at least one of said hollow cross-members, a pairof conduits connecting said hollow cross-members substantially inparallel, means to circulate cooling fluid through said hollowcross-members and said conduits to reduce the temperature of the rack toa predetermined level, a valve at one side of said hollow cross-membersto regulate the flow of cooling fluid, and means to maintain thetemperature of the rack accurately at said predetermined level bycontrolling the opening in said valve comprising a temperature-sensitiveelement in the form of an electrical resistor having a resistance whichvaries with temperature, an electric valve motor controlled by saidtemperature-sensitive element, and mechanical coupling means betweensaid valve motor and said valve, whereby the opening in said valvevaries with the resistance of said temperature-sensitive element.

10. An isothermal relay rack in accordance with claim 9 in which saidtemperature-sensitive element is connected as one of the resistance armsof a bridge circuit which is unbalanced in one direction when thetemperature of the rack is above said predetermined level and isunbalanced in the other direction when the tem- 11 perature of the rackis below said predetermined level.

11. An isothermal relay rack which comprises, in combination, a pair ofsupporting members, a plurality of substantially parallel flat,rectangular heat-conducting mounting plates extending between saidsupporting members, each of said mounting plates having a hollow raisedportion on one side thereof; forming a conduit extending substantiallythe entire distance between said supporting members, means to secure arow of electrical relays to each of said mounting plates with a directheat-conducting path between the core of each relay and the mountingplate to which it is secured, a pair of conduits extending alongrespective .ones of said supporting members, separate connections fromone end of the hollow raised portion of each of said mounting plates toone of said pair of conduits, separate connections from the other end ofthe hollow raised portion of each of said mounting plates to the otherofsaid pair of conduits, means to circulate cooling fluid from one end.of one of said pair of conduits to the, correspondingly opposite end ofthe other of said pair of conduits,

Wherebycooling. fluid is circulated through the hollow raised portion ofeach of said mounting plates and the temperature of said mounting platesis reduced to a predetermined value, and means to maintain thetemperature of said mounting plates accurately to said predeterminedlevel by regulating the flow of cooling fluid.

12. An isothermal relay rack in accordance with claim 11 in which all ofthe hollow raised portions of said mounting plates are on the oppositesides of said mounting plates from said pair of conduits.

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